(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
(2) Description of the Related Art
As measures taken against exhaust gas pollution and fuel consumption, a lean burn system has recently been developed. According to this lean burn system, a lean mixture sensor is provided for generating an analog current in proportion to the air-fuel mixture on the lean side in an exhaust pipe of an engine. Thus, the feedback of the air-fuel ratio of the engine can be controlled by using the analog output of the lean mixture sensor, thereby attaining any air-fuel ratio on the lean side.
That is, the above-mentioned air-fuel feedback control makes use of the characteristic strong relationship between the output current of a lean mixtue sensor to the concentration of the oxygen in the exhaust gas in the case of a lean air-fuel ratio, as compared with the stoichiometric air-fuel ratio.
However, the output current of the lean mixture sensor varies in accordance with the atmospheric pressure around the lean mixture sensor. For instance, at a high altitude, the atmospheric pressure is reduced. Accordingly, the output current of the lean mixture sensor is reduced, since this output current is approximately proportional to the oxygen concentration. As a result, the air-fuel ratio is sensed as on the richer side, whereby the air-fuel ratio feedback control controls the air-fuel ratio to be leaner. The actual air-fuel ratio thus becomes leaner with respect to the aimed lean air-fuel ratio, possibly causing misfiring or surging of the engine.